The invention relates to an high-speed optical switch having a digital operating mode in which the apparatus rapidly switches a light beam. When used to switch a light beam between two optical paths, the high-speed optical switch can be used for generating a light beam having rapidly alternating wavelengths or spectral bands, primarily for the purpose of alternating or interleaving the excitation or illumination energy delivered to a target. In addition to alternating the spectral content of the light beam, the high-speed optical switch is also capable of operating as a shuttering system to cut off the light beam completely. The high-speed optical switch may also be used to chop a light beam. The high-speed optical switch has an analog operating mode in which it can be used to control the intensity of a light beam. The high-speed optical switch may be used in both its digital and analog operating modes. For example, it may be used to generate a light beam with rapidly alternating spectral bands and to selectively attenuate or amplitude modulate the light beam in either or both of the alternating spectral bands.
The most commonly-used apparatus for generating a light beam having rapidly-alternating or sequencing spectral bands is the filter wheel shown in FIGS. 1A and 1B. A number of filters, such as the filters 11 and 13, are mounted on the filter wheel 15, which is caused to rotate by a suitable motor (not shown). The filters 11 and 13 select different spectral bands of the light from the light source 17. To reduce the speed at which the filter wheel is required to rotate for a given modulation frequency, nominally identical filters, such as the filters 11 and 12, are located at regular angular intervals on the filter wheel. Light from the light source 17 is focussed by the lens 19 on the proximal end 21 of the light fibre 23. The filter wheel is interposed between the lens 19 and the proximal end of the light fibre. In some systems, the filter wheel is actually a glass disk with different filter elements formed in or on the surface of the disk.
While the approach shown in FIGS. 1A and 1B is conceptually simple, is suffers from several drawbacks. High speed implementations (e.g., implementations providing alternation frequencies at or above the video frame rate of 30 Hz) may require that the rotating parts be carefully balanced to minimize vibration. Alternatively, the number of nominally identical filters may be increased reduce the rotational speed required for a given alternation frequency, but this requires that the nominally identical filters, e.g., 11 and 12, have accurately matched characteristics. Changing the spectral bands selected by the apparatus can be cumbersome, requiring that multiple filters 11, 12, 13, and 14 be carefully installed in the filter wheel to preserve the balance of the rotating system, or requiring that whole the filter wheel assembly, consisting of the filters 11-14 and the filter wheel 15, be changed. If multiple filters are used to reduce rotations speeds, providing a wide choice of spectral bands is expensive because a set of at least two matched filters is required for each possible spectral band. Compensation for variations in the matched filters in a set of filters may also be required.
The filter wheel systems shown in FIGS. 1A and 1B usually run at fixed speeds, so changing the effective exposure time of each phase of excitation is not possible. The angular momentum of the rotating filter wheel prevents instantaneous stopping or shuttering, and also makes synchronization to an external clock, e.g., a video frame rate clock, difficult. Finally, because the edge of each filter progressively moves across the width of the light beam as the filter wheel rotates, the on-off transitions between the spectral bands is not abrupt, and the transitions may include a fixed period of no illumination, the duration of which may not be optimum for all applications. This results is a reduction of the duty cycle and loss of efficiency in energy delivery. In practice, commercial filter wheels designed for interchangeable filter elements are usually restricted to manual or slow speed (several Hertz) operation.
A second method of switching wavelengths is to use a motorized monochrometer. In this, the angle of a diffraction grating, which selects a narrow spectral band from a light source, or the angle of incidence of light on the diffraction grating, is modulated to change the selected spectral band. Some of these systems claim speeds of, or greater than, 30 Hz, but the light beam generated by such systems usually has a lower intensity than that of the light beam generated by filter-based systems, such as the filter-wheel systems just described. Monochrometers usually have a high f-number, which results in a poor optical coupling efficiency. Moreover, the spectral band selected by a monochrometer is normally narrow, in the range of 1-10 nm. Filter-based systems, on the other hand, usually have a low f-number, and therefore have a high optical coupling efficiency. Filter-based systems may be more versatile, and allow the interchangeable use of narrow-band, broad-band, or even more sophisticated multi-band filter elements. Filter-based systems provide greater ease of used in applications in which a relatively few spectral bands are consistently required.
An alternative filter-based system, which is a variation on the filter wheel system shown in FIGS. 1A and 1B is shown in FIGS. 1C and 1D. The variation shown in FIGS. 1C and 1D provides the advantages of a filter-based system discussed above, while overcoming some of the principal shortcomings of the filter wheel shown in FIGS. 1A and 1B. In the slotted-mirrored wheel system shown in FIGS. 1C and 1D, the light from a light source (not shown) is divided into first and second light beams 31 and 33, respectively, which are orthogonal to one another. The wavelength or spectral band-selective filter 35 is mounted in the first light beam 31, and the lens 39 focuses the filtered first light beam on the proximal end 21 of the light fibre 23. The wavelength or spectral band-selective filter 37 is mounted in the second light beam 33, and the lens 41 focuses the filtered second light beam on the proximal end 21 of the light fibre 23.
The slotted-mirrored wheel 43 is mounted in a plane at 45 degrees to both the first light beam 31 and the second light beam 33 to select one of the two light beams, or the other, to illuminate the proximal end 21 of the light fibre 23. The slotted-mirrored wheel includes holes, such as the holes 45 and 47 at regular angular spacings, which select the first light beam by passing the first light beam to illuminate the proximal end of the light fibre. Mounted on the slotted-mirrored wheel at regular angular intervals are the mirrors 49 and 51, which reflect the second light beam through an angle of 90.degree. to illuminate the proximal end of the light fibre.
With this approach, it is easier to use multiple mirrors and holes to reduce the rotational speed of the slotted-mirrored wheel required to provide a given alternation frequency because the problems of matching the mirror characteristics are less severe. Moreover, changing the filters is simpler, and the cost of the filters required to provide a wide selection of spectral bands is less. However, the slotted-mirrored wheel still suffers from the above-described limitations resulting from the large angular momentum and inertia of a wheel-based system.
It is also known to use liquid filter technology to activate different filter layers sandwiched in a filter stack. However, this method is inflexible, because the filter selection is fixed once the assembly is constructed, and is limited to the visible region of the spectrum.
Finally, it is known to use two light sources with filters selecting different spectral bands or wavelengths, and to combine the light beams generated by the two light sources. Wavelength or spectral band alternation is provides by alternately switching the power supplies to the light sources on and off. This approach is relatively expensive, because it requires two light sources, and also requires special switchable power supplies for the light sources.